 
  

 






<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 
<html>

<!-- Mirrored from www.javapractices.com/topic/TopicAction.do?Id=215 by HTTrack Website Copier/3.x [XR&CO'2010], Sun, 12 Jun 2011 17:28:08 GMT -->
<!-- Added by HTTrack --><meta http-equiv="content-type" content="text/html;charset=UTF-8"><!-- /Added by HTTrack -->
<head>
 <title>
  Java Practices -> Constructors shouldn't call overridables
 </title>
 <link rel="stylesheet" type="text/css" href="../stylesheet8.css" media="all">
 
 <link rel="shortcut icon" href='../images/favicon.ico' type="image/vnd.microsoft.icon">
 <meta name="description" content="Concise presentations of java programming practices, tasks, and conventions, amply illustrated with syntax highlighted code examples.">
 
 <meta name='keywords' content='java,java programming,java practices,java idiom,java style,java design patterns,java coding conventions,'>
 
 
</head>
 
<body>


<div class='menu-bar'>
 
  <a href='../home/HomeAction.html' title='Table of Contents'>Home</a> |
  <a href='../vote/VoteSummaryAction-2.html' title='View Poll Results'>Poll</a> |
   
  <A href='../feedback/FeedbackAction451f-2.html?Operation=Show' title='Send Your Feedback'>Wiki</a> |
  <b><a href='../source/SourceAction-2.html' title='Grab Source Code'>Source Code</a></b><IMG class='no-margin' SRC="../images/goldstar.gif" ALT=""> |

  <a href='http://www.web4j.com/Java_Web_Application_Framework_Overview.jsp?From=1' title='Free Download - Java Web Application Framework'><b>WEB4J</b></a> |
  
  <a href='http://www.date4j.net/' title='Replacement for java.util.Date'><b>DATE4J</b></a> |

   <a href='../references/ReferencesAction-2.html' title='References'>Links</a>
   
  <form action='http://www.javapractices.com/search/SearchAction.do' method='get' class='search-form'>
   <input type='text' name='SearchTerms' value="" size=12 maxlength=50 class='search'>
   <input type='submit' value="Search">
  </form>
 
</div>

<P>



  

 






<p class="display-messages">

 

 

</p>


<div class="main-layout">
 
   

 




<div class='page-title'>Constructors shouldn't call overridables</div>

<div class='main-body'>
 <br>
Constructors should not call an overridable method - that is, they should only call methods 
that are <tt>private</tt>, <tt>static</tt>, or <tt>final</tt>.

<P>The reason why is best illustrated with a bit of code :
<br>
<PRE>

<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> ConstructorOverridable {

  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>void</span> main(String... aArgs){
    log(<span class='literal'>"BUILD AN ANIMAL :"</span>);
    Animal animal = <span class='keyword'>new</span> Animal();
    log(<span class='literal'>" "</span>);
    log(<span class='literal'>"BUILD A FISH :"</span>);
    Animal fish = <span class='keyword'>new</span> Fish();
  }
  
  <span class='comment'>// PRIVATE //
</span>  <span class='keyword'>private</span> <span class='keyword'>static</span> <span class='keyword'>void</span> log(Object aObject){
    System.out.println(String.valueOf(aObject));
  }
  
  <span class='keyword'>private</span> <span class='keyword'>static</span> <span class='keyword'>class</span> Animal{
    Animal(){
      log(<span class='literal'>"Animal ctor"</span>);
      <span class='comment'>//call an overridable method from ctor - dangerous
</span>      perform();
    }
    <span class='keyword'>void</span> perform(){
      log(<span class='literal'>"Animal perform."</span>);
    }
  }
  
  <span class='keyword'>private</span> <span class='keyword'>static</span> <span class='keyword'>class</span> Fish <span class='keyword'>extends</span> Animal {
    Fish(){
      <span class='comment'>//every constructor has an implicit call to super(); for first line 
</span>      <span class='keyword'>super</span>();
      log(<span class='literal'>"Fish ctor. This is the earliest Tail Shape can be initialized."</span>);
    }
    <span class='keyword'>void</span> perform(){
      <span class='comment'>//in general, this method can depend on Fish data
</span>      log(<span class='literal'>"Fish perform. Can fail since Tail Shape is : "</span> + fTailShape);
    }
    <span class='keyword'>private</span> String fTailShape;
  }
}
 
</PRE>
<br>
An example run of this class demonstrates the problem :
<PRE>
BUILD AN ANIMAL :
Animal ctor
Animal perform.
 
BUILD A FISH :
Animal ctor
Fish perform. Can fail since Tail Shape is : null
Fish ctor. This is the earliest Tail Shape can be initialized.
</PRE>
<br>

</div>




<div class='topic-section'>See Also :</div>
<div class='main-body'>
 
  
  <a href='TopicAction19ff-2.html?Id=11'>Constructors in general</a> <br>
 
  
  <a href='TopicAction18c3-2.html?Id=89'>Overridable methods need special care</a> <br>
 
</div>


<div class='topic-section'>Would you use this technique?</div>
<div class='main-body'>
  
  <form action="http://www.javapractices.com/vote/AddVoteAction.do" method='post'>
    Yes<input type='radio' name='Choice' value='Y' >
    &nbsp;&nbsp;No<input type='radio' name='Choice' value='N'>
    &nbsp;&nbsp;Undecided<input type='radio' name='Choice' value="?" >
    &nbsp;&nbsp;<input type=submit value="Vote" >
    <input type='hidden' name='Operation' value='Apply'>
    <input type='hidden' name='TopicId' value='215'>
  </form>
</div>

<div style='height:10.0em;'></div>

 
 
</div>

  

 





<div align='center' class='legalese'>  
&copy; 2011 Hirondelle Systems |
<a href='../source/SourceAction-2.html'><b>Source Code</b></a><IMG class='no-margin' SRC="../images/goldstar.gif" ALT=""> |
<a href="mailto:webmaster@javapractices.com">Contact</a> |
<a href="http://creativecommons.org/licenses/by-nc-sa/1.0/">License</a> |
<a href='../apps/cjp.rss'>RSS</a>
<!-- ukey="2AC36CD2" -->
<!-- ckey="16DF3D87" -->
<br>

 Individual code snippets can be used under this <a href='../LICENSE.txt'>BSD license</a> - Last updated on June 6, 2010.<br>
 Over 150,000 unique IPs last month - <span title='Java Practices 2.6.5, Mon May 16 00:00:00 EDT 2011'>Built with</span> <a href='http://www.web4j.com/'>WEB4J</a>.<br>
 - In Memoriam : Bill Dirani -
</div>

<script src="../../www.google-analytics.com/urchin.js" type="text/javascript">
</script>
<script type="text/javascript">
_uacct = "UA-2633428-1";
urchinTracker();
</script>



</body>

<!-- Mirrored from www.javapractices.com/topic/TopicAction.do?Id=215 by HTTrack Website Copier/3.x [XR&CO'2010], Sun, 12 Jun 2011 17:28:08 GMT -->
<!-- Added by HTTrack --><meta http-equiv="content-type" content="text/html;charset=UTF-8"><!-- /Added by HTTrack -->
</html>
